Audio Signal Limiter

ABSTRACT

A computing device is configured to (i) receive an indication of an operational limit of a given playback device, where the given playback device is included in a group of at least two playback devices that are configured for synchronous playback of audio content, (ii) receive a first audio signal for playback by the group of at least two playback devices, (iii) based on the operational limit of the given playback device, apply a filter to the first audio signal to generate a second audio signal, and (iv) transmit the second audio signal to at least the given playback device for synchronous playback by the group of at least two playback devices.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 120 to, and is acontinuation of U.S. patent application Ser. No. 17/063,189, filed onOct. 5, 2020 and entitled “Limiter for Bass Enhancement,” which is acontinuation of U.S. patent application Ser. No. 16/146,047 filed onSep. 28, 2018, now U.S. Pat. No. 10,798,486, entitled “Limiter for BassEnhancement,” which is a continuation of U.S. patent application Ser.No. 15/858,202 filed on Dec. 29, 2017, now U.S. Pat. No. 10,123,118,entitled “Limiter for Bass Enhancement,” which is a continuation of U.S.patent application Ser. No. 15/479,458 filed on Apr. 5, 2017, now U.S.Pat. No. 9,860,644, entitled “Limiter for Bass Enhancement,” thecontents of each of which are hereby incorporated by reference in theirentirety for all purposes.

FIELD OF THE DISCLOSURE

The disclosure is related to consumer goods and, more particularly, tomethods, systems, products, features, services, and other elementsdirected to media playback or some aspect thereof.

BACKGROUND

Options for accessing and listening to digital audio in an out-loudsetting were limited until 2003, when SONOS, Inc. filed for one of itsfirst patent applications, entitled “Method for Synchronizing AudioPlayback between Multiple Networked Devices,” and began offering a mediaplayback system for sale in 2005. The Sonos Wireless HiFi System enablespeople to experience music from many sources via one or more networkedplayback devices. Through a software control application installed on asmartphone, tablet, or computer, one can play what he or she wants inany room that has a networked playback device. Additionally, using thecontroller, for example, different songs can be streamed to each roomwith a playback device, rooms can be grouped together for synchronousplayback, or the same song can be heard in all rooms synchronously.

Given the ever-growing interest in digital media, there continues to bea need to develop consumer-accessible technologies to further enhancethe listening experience.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and advantages of the presently disclosed technologymay be better understood with regard to the following description,appended claims, and accompanying drawings where:

FIG. 1 shows an example media playback system configuration in whichcertain embodiments may be practiced;

FIG. 2 shows a functional block diagram of an example playback device;

FIG. 3 shows a functional block diagram of an example control device;

FIG. 4 shows an example controller interface;

FIG. 5 shows an example plurality of network devices;

FIG. 6 shows a functional block diagram of an example network microphonedevice;

FIG. 7 shows an example input and output associated with an examplelimiter for an audio playback environment;

FIG. 8 shows example functions associated with an example dynamicsidechain filter for an audio playback environment;

FIG. 9 shows an example of a low-band reduction filter;

FIG. 10 shows an example of a mid-band reduction filter;

FIG. 11 shows an example of a wide-shelf reduction filter;

FIG. 12 shows an example comparison of a dry audio signal with aperformance ceiling response associated with an audio playback pipeline;

FIG. 13 shows a flow chart of functions associated with the exampledynamic sidechain filter; and

FIG. 14 shows an example arrangement of a limiter.

The drawings are for purpose of illustrating example embodiments, but itis understood that the inventions are not limited to the arrangementsand instrumentality shown in the drawings.

DETAILED DESCRIPTION I. Overview

An audio signal may be input into an audio playback device and the audioplayback device may output audio based on the audio signal. The audioplayback device may then play audio content at different volumesettings. Further, the bass may be boosted at the different volumes tocorrect for a natural response (EQ) of a speaker and/or acoustics ofroom and/or the content. A bass response of the audio playback devicemay be boosted by increasing a low frequency gain of the audio signalplayed back by the audio playback device. However, an operational limitof the audio playback device may determine whether, with bass boosting,the audio is played back without distortion and/or without damaging theaudio playback device as higher volume settings are reached.

The operational limit may take a variety of forms. For example, theoperational limit might include a maximum excursion of a transducer.Additionally, or alternatively, the operational limit may includevoltage, current, power, and temperature limits of an audio amplifier.With bass boosting, the audio playback device could approach theoperational limit before a volume setting approaches a highest volumesetting. To maintain operation within the operational limit as thevolume continues to be increased and maximizing perceived loudness, anamplitude of the audio signal may be limited at low frequencies toprevent the excursion and/or voltage, current, power, temperature, orany other physical parameters from exceeding the operational limit anddamaging the playback device. The limiting may allow the playback deviceto reach higher volumes than physically permissible without distortion.The limiting may take a variety of forms applied in a variety ofcombinations.

In one example, the audio signal with increased gain may be passedthrough a series of analog limiters. The analog limiters include variousanalog electrical components such as capacitors, resistors, andinductors. The analog limiters may track a level of an audio signal(e.g., RMS level) in the form of an analog signal and prevent the levelof the audio signal from exceeding a defined threshold. The analoglimiters attenuate an amplitude of the audio signal at one or morefrequencies so that the audio playback device continues to operatewithin its operational limit.

In another example, the audio signal with increased gain may be passedthrough one or more digital filters. The digital filter may take theform of a high pass filter and/or low frequency (e.g., bass) filter withgain and cutoff frequencies that change as a function of one or more oftime, the audio signal, and/or a previous gain and/or cutoff frequency.The digital filter attenuates an amplitude of the audio signal at one ormore frequencies so that the audio playback device continues to operatewithin its operational limit. Typically, this attenuation isaccomplished by dynamically adjusting a threshold frequency, Q, and/orgain of a digital filter via filter coefficients of the digital filter.

In yet another example, the audio signal with increased gain may beanalyzed to estimate an expected excursion. For example, the excursionmay be how far a cone of the transducer travels from a resting positionwhen an audio signal after equalization and volume gain is applied tothe transducer. An excursion model may be used to estimate this expectedexcursion. The excursion model may take as inputs known characteristicsof the transducer and the audio signal to predict the excursion. Basedon the amount of excursion, a threshold frequency, Q, and/or gain of adigital filter can be adjusted so that the transducer component operateswithin its operational limit as the audio signal is passed through thedigital filter. Other physical parameters such as power, voltage, andcurrent can also be modeled like excursion and used to dynamicallyadjust the threshold frequency, Q, and/or gain of the digital filter.

In another example, components of an audio playback device may bedesigned to have certain mechanical properties. For example, a spidermay have a certain elasticity or rigidity similar to that of a springwhich cause a restorative force to be applied to the cone. Themechanical properties may prevent transducer components from beingdamaged. The spider may have other types of restorative properties aswell.

A problem with conventional methods of maintaining operation of theaudio playback devices within an operational limit is that it canproduce a choppy listening experience. For example, in certain bassheavy content (e.g., acoustic content with a lot of bass such as a kickdrum), the loudness of the kick drum may be inconsistent as thethreshold frequency, Q, and/or gain of the digital filter dynamicallychanges to maintain operation of the audio playback device within theoperational limit. Quieter beats of the kickdrum may be limited lesswhereas louder beats of the kick drum (which are expected to exceed thetransducer's physical limits) may be limited more. The difference inlimiting may result in the bass appearing to unnaturally cut in and out.

Embodiments described herein are directed to a limiter which reduces thechoppiness associated with conventional limiting schemes. The audioplayback device may operate within operational limit without producingthe choppiness that would otherwise be perceivable or minimallyperceivable as distortion. Also, stresses on the audio playback devicemay not increase.

The audio signal played by the audio playback device may take a varietyof forms. In one example, the audio signal may be a digital audio signalsuch as a packetized or non-packetized stream of audio from a musicservice or television, a digital audio file, or audio signals generatedby the audio playback device itself or a device connected to the audioplayback device. In another example, the audio signal may be analogsignal input from an auxiliary connection or a digital signal input froma USB connection. The audio signal may comprise frequency content thatrange from 0 Hz to 22,050 Hz or some subset of this frequency range. Theaudio signal may be input into one or more dynamic sidechain filters(DSF).

The DSF may store or receive from a network device an indication of anoperational limit of the playback device. The indication may take theform of a threshold. In some examples, the threshold may be aperformance ceiling response which defines maximum amplitudes of anaudio signal input into an audio playback pipeline over a range offrequencies so that the audio playback pipeline does not exceed anoperational limit. The operational limit may include various physicallimits, such as electrical, mechanical, and/or thermal limits associatedwith the audio playback pipeline as well.

The DSF may compare an envelope of a dry signal with the threshold. Thedry signal may be an audio signal after amplification to account for avolume setting but before being converted to an analog signal andamplified for output by the transducer. Additionally, or alternatively,the dry signal may be an audio signal processed by an equalizer. Anoutput of the comparison may be a dry excess corresponding to anenvelope of the dry signal in excess of the threshold. Similarly, theDSF may compare an envelope of a wet signal to the threshold. The wetsignal may be the result of applying a filter to the dry audio signal.An output of the comparison may be a wet excess corresponding to anenvelope of the wet signal in excess of the threshold.

The filter applied to the dry signal to produce the wet signal may be afixed filter which filters the audio signal in a manner which ispleasing to a listener while conforming to the limits of the audioplayback pipeline. The filter may take the form of a finite impulseresponse filter (FIR), warped FIR, infinite impulse response filter(IIR), among others. The filter may allow for operational limit to bereached most readily in the low frequencies and with minimal perceptionby the listener.

The wet excess and dry excess may be used to determine how much the dryand wet signals are to be mixed to maintain operation of the audioplayback device within an operational limit while affecting the drysignal as little as possible. A gain for the wet signal and a gain forthe dry signal may be calculated based on the wet excess and the dryexcess. Then, the dry signal and wet signal may be mixed in proportionto respective gains to output a mixed signal.

The DSF may act as a limiter. In this regard, the audio playback devicewhich plays back the mixed signal may operate within operational limits.Alternatively, the mixed signal may be input into one or more additionalinstances of a DSF. The two or more DSFs may be cascaded together suchthat an output of one DSF is an input to another DSF. Each DSF may applya given filter such that the cascade of DSFs acts as a limiter.Properties of the given filter of each DSF and/or an order in which thefilters are applied (via the cascade of DSFs) may determine propertiesof the limiter. An output of the cascade of DSFs may then be played backby the audio playback device.

To illustrate, an audio signal may be received by an audio playbackdevice. The audio signal may be a digital signal sampled at a given rateand streamed to the audio playback device. Alternatively, the signal maybe an analog audio signal. In some cases, the audio playback device mayhave performed some type of signal processing on the audio signal suchas equalization to boost desired spectral content or addition of aconstant gain to the audio signal for volume amplification. The DSF maytake as input the audio signal and generate a dry and wet signal. Thedry and wet signals may be mixed in proportion in various frequencybands. For example, the wet signal may be 2 dB (avg.) below aperformance ceiling response which takes the form of an excursionthreshold in the 30-50 Hz band. The dry signal may be 10 dB above theexcursion threshold in the frequency band. This means that the drysignal which is 10 dB above the excursion threshold may need to bereduced by 8 dB for an audio signal in the frequency band to exist belowthe threshold value, ˜80% of the wet signal will be mixed with ˜20% ofthe dry signal. The mixed signal output by the DSF may then be playedback or input into cascade of two or more DSFs which output a mixedsignal. The mixed signal from the DSF or cascade of DSFs may prevent theaudio playback device from exceeding an operational limit, which wouldotherwise happen if the audio signal was not processed by the describedDSF or cascade of DSFs.

Moving on from the above illustration, an example embodiment may be aplayback device comprising a processor; memory; and computerinstructions stored in the memory and executable by the processor tocause the processor to receive a first audio signal; compare an envelopeof the first audio signal to a threshold to determine a first excessindicative of an amount by which the envelope of the first audio signalexceeds the threshold; apply a filter to the first audio signal togenerate a second audio signal; compare an envelope of the second audiosignal to the threshold to determine a second excess indicative of anamount by which the envelope of the second audio signal exceeds thethreshold; based on the first excess and the second excess, determine afirst gain associated with the first audio signal and a second gainassociated with the second audio signal; mix the first audio signal andthe second audio signal based on the first gain and the second gain tooutput a mixed signal; and cause play back of audio based on the mixedsignal.

Applying the filter to the first audio signal to generate the secondaudio signal may comprise attenuating an amplitude of the first audiosignal in a frequency range. Applying the filter to the first audiosignal to generate the second audio signal may further comprise boostingan amplitude of the first audio signal outside of the frequency range.The frequency range may be within a 0 to 100 Hz range. The playbackdevice may further comprise computer instructions for determining theenvelope of the first audio signal and the second audio signal. Thethreshold of the playback device may be associated with an operationallimit which over which damage to the playback device may occur. Theplayback device may further comprise computer instructions for selectingthe filter based on a frequency range played back by the playbackdevice. Applying a filter to the first audio signal to generate a secondaudio signal may comprise applying the filter to a first band of thefirst audio signal and then applying the filter to a second band of thefirst audio signal, where the first band and second band arenon-overlapping frequency bands. The computer instructions stored in thememory and executable by the processor may define a DSF. Causing theplay back of audio based on the mixed signal may comprise inputting themixed signal to a second DSF different from the first DSF.

Another example embodiment may be a method comprising receiving a firstaudio signal; comparing an envelope of the first audio signal to athreshold to determine a first excess indicative of an amount by whichthe envelope of the first audio signal exceeds the threshold; applying afilter to the first audio signal to generate a second audio signal;comparing an envelope of the second audio signal to the threshold todetermine a second excess indicative of an amount by which the envelopeof the second audio signal exceeds the threshold; based on the firstexcess and the second excess, determining a first gain associated withthe first audio signal and a second gain associated with the secondaudio signal; mixing the first audio signal and the second audio signalbased on the first gain and the second gain to output a mixed signal;and playing back audio based on the mixed signal.

Applying the filter to the first audio signal to generate the secondaudio signal may comprise attenuating an amplitude of the first audiosignal in a frequency range. Applying the filter to the first audiosignal to generate the second audio signal may comprise boosting anamplitude of the first audio signal outside of the frequency range. Thefrequency range may be within a 0 to 100 Hz range. The first excess andthe second excess may be positive or negative. The method may furthercomprise selecting the filter based on a frequency range played back bythe playback device. Applying a filter to the first audio signal togenerate a second audio signal may comprise applying the filter to afirst band of the first audio signal and then applying the filter to asecond band of the first audio signal, where the first band and secondband are non-overlapping frequency bands.

Yet another example embodiment may be a tangible non-transitory computerreadable storage medium including instructions for execution by aprocessor, the instructions, when executed, cause the processor toimplement a method comprising receiving a first audio signal; comparingan envelope of the first audio signal to a threshold to determine afirst excess indicative of an amount by which the envelope of the firstaudio signal exceeds the threshold; applying a filter to the first audiosignal to generate a second audio signal; comparing an envelope of thesecond audio signal to the threshold to determine a second excessindicative of an amount by which the envelope of the second audio signalexceeds the threshold; based on the first excess and the second excess,determining a first gain associated with the first audio signal and asecond gain associated with the second audio signal; mixing the firstaudio signal and the second audio signal based on the first gain and thesecond gain to output a mixed signal; and playing back audio based onthe mixed signal.

Applying the filter to the first audio signal to generate the secondaudio signal may comprise attenuating an amplitude of the first audiosignal in a frequency range. Applying the filter to the first audiosignal to generate the second audio signal may comprise boosting anamplitude of the first audio signal outside of the frequency range. Thefrequency range may be within a 0 to 100 Hz range.

While some examples described herein may refer to functions performed bygiven actors such as “users” and/or other entities, it should beunderstood that this is for purposes of explanation only. The claimsshould not be interpreted to require action by any such example actorunless explicitly required by the language of the claims themselves. Itwill be understood by one of ordinary skill in the art that thisdisclosure includes numerous other embodiments. Moreover, the examplesdescribed herein may extend to a multitude of embodiments formed bycombining the example features in any suitable manner.

II. Example Operating Environment

FIG. 1 shows an example configuration of a media playback system 100 inwhich one or more embodiments disclosed herein may be practiced orimplemented. The media playback system 100 as shown is associated withan example home environment having several rooms and spaces, such as forexample, a master bedroom, an office, a dining room, and a living room.As shown in the example of FIG. 1, the media playback system 100includes playback devices 102-124, control devices 126 and 128, and awired or wireless network router 130.

Further discussions relating to the different components of the examplemedia playback system 100 and how the different components may interactto provide a user with a media experience may be found in the followingsections. While discussions herein may generally refer to the examplemedia playback system 100, technologies described herein are not limitedto applications within, among other things, the home environment asshown in FIG. 1. For instance, the technologies described herein may beuseful in environments where multi-zone audio may be desired, such as,for example, a commercial setting like a restaurant, mall or airport, avehicle like a sports utility vehicle (SUV), bus or car, a ship or boat,an airplane, and so on.

a. Example Playback Devices

FIG. 2 shows a functional block diagram of an example playback device200 that may be configured to be one or more of the playback devices102-124 of the media playback system 100 of FIG. 1. The playback device200 may include a processor 202, software components 204, memory 206,audio processing components 208, audio amplifier(s) 210, speaker(s) 212,a network interface 214 including wireless interface(s) 216 and wiredinterface(s) 218, and microphone(s) 220. In one case, the playbackdevice 200 may not include the speaker(s) 212, but rather a speakerinterface for connecting the playback device 200 to external speakers.In another case, the playback device 200 may include neither thespeaker(s) 212 nor the audio amplifier(s) 210, but rather an audiointerface for connecting the playback device 200 to an external audioamplifier or audio-visual receiver.

In one example, the processor 202 may be a clock-driven computingcomponent configured to process input data according to instructionsstored in the memory 206. The memory 206 may be a tangiblecomputer-readable medium configured to store instructions executable bythe processor 202. For instance, the memory 206 may be data storage thatcan be loaded with one or more of the software components 204 executableby the processor 202 to achieve certain functions. In one example, thefunctions may involve the playback device 200 retrieving audio data froman audio source or another playback device. In another example, thefunctions may involve the playback device 200 sending audio data toanother device or playback device on a network. In yet another example,the functions may involve pairing of the playback device 200 with one ormore playback devices to create a multi-channel audio environment.

Certain functions may involve the playback device 200 synchronizingplayback of audio content with one or more other playback devices.During synchronous playback, a listener will preferably not be able toperceive time-delay differences between playback of the audio content bythe playback device 200 and the one or more other playback devices. U.S.Pat. No. 8,234,395 entitled, “System and method for synchronizingoperations among a plurality of independently clocked digital dataprocessing devices,” which is hereby incorporated by reference, providesin more detail some examples for audio playback synchronization amongplayback devices.

The memory 206 may further be configured to store data associated withthe playback device 200, such as one or more zones and/or zone groupsthe playback device 200 is a part of, audio sources accessible by theplayback device 200, or a playback queue that the playback device 200(or some other playback device) may be associated with. The data may bestored as one or more state variables that are periodically updated andused to describe the state of the playback device 200. The memory 206may also include the data associated with the state of the other devicesof the media system, and shared from time to time among the devices sothat one or more of the devices have the most recent data associatedwith the system. Other embodiments are also possible.

The audio processing components 208 may include one or moredigital-to-analog converters (DAC), an audio preprocessing component, anaudio enhancement component or a digital signal processor (DSP), and soon. In one embodiment, one or more of the audio processing components208 may be a subcomponent of the processor 202. In one example, audiocontent may be processed and/or intentionally altered by the audioprocessing components 208 to produce audio signals. The produced audiosignals may then be provided to the audio amplifier(s) 210 foramplification and playback through speaker(s) 212. Particularly, theaudio amplifier(s) 210 may include devices configured to amplify audiosignals to a level for driving one or more of the speakers 212. Thespeaker(s) 212 may include an individual transducer (e.g., a “driver”)or a complete speaker system involving an enclosure with one or moredrivers. A particular driver of the speaker(s) 212 may include, forexample, a subwoofer (e.g., for low frequencies), a mid-range driver(e.g., for middle frequencies), and/or a tweeter (e.g., for highfrequencies). In some cases, each transducer in the one or more speakers212 may be driven by an individual corresponding audio amplifier of theaudio amplifier(s) 210. In addition to producing analog signals forplayback by the playback device 200, the audio processing components 208may be configured to process audio content to be sent to one or moreother playback devices for playback.

Audio content to be processed and/or played back by the playback device200 may be received from an external source, such as via an audioline-in input connection (e.g., an auto-detecting 3.5 mm audio line-inconnection) or the network interface 214.

The network interface 214 may be configured to facilitate a data flowbetween the playback device 200 and one or more other devices on a datanetwork. As such, the playback device 200 may be configured to receiveaudio content over the data network from one or more other playbackdevices in communication with the playback device 200, network deviceswithin a local area network, or audio content sources over a wide areanetwork such as the Internet. In one example, the audio content andother signals transmitted and received by the playback device 200 may betransmitted in the form of digital packet data containing an InternetProtocol (IP)-based source address and IP-based destination addresses.In such a case, the network interface 214 may be configured to parse thedigital packet data such that the data destined for the playback device200 is properly received and processed by the playback device 200.

As shown, the network interface 214 may include wireless interface(s)216 and wired interface(s) 218. The wireless interface(s) 216 mayprovide network interface functions for the playback device 200 towirelessly communicate with other devices (e.g., other playbackdevice(s), speaker(s), receiver(s), network device(s), control device(s)within a data network the playback device 200 is associated with) inaccordance with a communication protocol (e.g., any wireless standardincluding IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4Gmobile communication standard, and so on). The wired interface(s) 218may provide network interface functions for the playback device 200 tocommunicate over a wired connection with other devices in accordancewith a communication protocol (e.g., IEEE 802.3). While the networkinterface 214 shown in FIG. 2 includes both wireless interface(s) 216and wired interface(s) 218, the network interface 214 may in someembodiments include only wireless interface(s) or only wiredinterface(s).

The microphone(s) 220 may be arranged to detect sound in the environmentof the playback device 200. For instance, the microphone(s) may bemounted on an exterior wall of a housing of the playback device. Themicrophone(s) may be any type of microphone now known or later developedsuch as a condenser microphone, electret condenser microphone, or adynamic microphone. The microphone(s) may be sensitive to a portion ofthe frequency range of the speaker(s) 220. One or more of the speaker(s)220 may operate in reverse as the microphone(s) 220. In some aspects,the playback device 200 might not include the microphone(s) 220.

In one example, the playback device 200 and one other playback devicemay be paired to play two separate audio components of audio content.For instance, playback device 200 may be configured to play a leftchannel audio component, while the other playback device may beconfigured to play a right channel audio component, thereby producing orenhancing a stereo effect of the audio content. The paired playbackdevices (also referred to as “bonded playback devices”) may further playaudio content in synchrony with other playback devices.

In another example, the playback device 200 may be sonicallyconsolidated with one or more other playback devices to form a single,consolidated playback device. A consolidated playback device may beconfigured to process and reproduce sound differently than anunconsolidated playback device or playback devices that are paired,because a consolidated playback device may have additional speakerdrivers through which audio content may be rendered. For instance, ifthe playback device 200 is a playback device designed to render lowfrequency range audio content (i.e. a subwoofer), the playback device200 may be consolidated with a playback device designed to render fullfrequency range audio content. In such a case, the full frequency rangeplayback device, when consolidated with the low frequency playbackdevice 200, may be configured to render only the mid and high frequencycomponents of audio content, while the low frequency range playbackdevice 200 renders the low frequency component of the audio content. Theconsolidated playback device may further be paired with a singleplayback device or yet another consolidated playback device.

By way of illustration, SONOS, Inc. presently offers (or has offered)for sale certain playback devices including a “PLAY:1,” “PLAY:3,”“PLAY:5,” “PLAYBAR,” “CONNECT:AMP,” “CONNECT,” and “SUB.” Any otherpast, present, and/or future playback devices may additionally oralternatively be used to implement the playback devices of exampleembodiments disclosed herein. Additionally, it is understood that aplayback device is not limited to the example illustrated in FIG. 2 orto the SONOS product offerings. For example, a playback device mayinclude a wired or wireless headphone. In another example, a playbackdevice may include or interact with a docking station for personalmobile media playback devices. In yet another example, a playback devicemay be integral to another device or component such as a television, alighting fixture, or some other device for indoor or outdoor use.

b. Example Playback Zone Configurations

Referring back to the media playback system 100 of FIG. 1, theenvironment may have one or more playback zones, each with one or moreplayback devices. The media playback system 100 may be established withone or more playback zones, after which one or more zones may be added,or removed to arrive at the example configuration shown in FIG. 1. Eachzone may be given a name according to a different room or space such asan office, bathroom, master bedroom, bedroom, kitchen, dining room,living room, and/or balcony. In one case, a single playback zone mayinclude multiple rooms or spaces. In another case, a single room orspace may include multiple playback zones.

As shown in FIG. 1, the balcony, dining room, kitchen, bathroom, office,and bedroom zones each have one playback device, while the living roomand master bedroom zones each have multiple playback devices. In theliving room zone, playback devices 104, 106, 108, and 110 may beconfigured to play audio content in synchrony as individual playbackdevices, as one or more bonded playback devices, as one or moreconsolidated playback devices, or any combination thereof. Similarly, inthe case of the master bedroom, playback devices 122 and 124 may beconfigured to play audio content in synchrony as individual playbackdevices, as a bonded playback device, or as a consolidated playbackdevice.

In one example, one or more playback zones in the environment of FIG. 1may each be playing different audio content. For instance, the user maybe grilling in the balcony zone and listening to hip hop music beingplayed by the playback device 102 while another user may be preparingfood in the kitchen zone and listening to classical music being playedby the playback device 114. In another example, a playback zone may playthe same audio content in synchrony with another playback zone. Forinstance, the user may be in the office zone where the playback device118 is playing the same rock music that is being playing by playbackdevice 102 in the balcony zone. In such a case, playback devices 102 and118 may be playing the rock music in synchrony such that the user mayseamlessly (or at least substantially seamlessly) enjoy the audiocontent that is being played out-loud while moving between differentplayback zones. Synchronization among playback zones may be achieved ina manner similar to that of synchronization among playback devices, asdescribed in previously referenced U.S. Pat. No. 8,234,395.

As suggested above, the zone configurations of the media playback system100 may be dynamically modified, and in some embodiments, the mediaplayback system 100 supports numerous configurations. For instance, if auser physically moves one or more playback devices to or from a zone,the media playback system 100 may be reconfigured to accommodate thechange(s). For instance, if the user physically moves the playbackdevice 102 from the balcony zone to the office zone, the office zone maynow include both the playback device 118 and the playback device 102.The playback device 102 may be paired or grouped with the office zoneand/or renamed if so desired via a control device such as the controldevices 126 and 128. On the other hand, if the one or more playbackdevices are moved to a particular area in the home environment that isnot already a playback zone, a new playback zone may be created for theparticular area.

Further, different playback zones of the media playback system 100 maybe dynamically combined into zone groups or split up into individualplayback zones. For instance, the dining room zone and the kitchen zone114 may be combined into a zone group for a dinner party such thatplayback devices 112 and 114 may render audio content in synchrony. Onthe other hand, the living room zone may be split into a television zoneincluding playback device 104, and a listening zone including playbackdevices 106, 108, and 110, if the user wishes to listen to music in theliving room space while another user wishes to watch television.

c. Example Control Devices

FIG. 3 shows a functional block diagram of an example control device 300that may be configured to be one or both of the control devices 126 and128 of the media playback system 100. As shown, the control device 300may include a processor 302, memory 304, a network interface 306, a userinterface 308, microphone(s) 310, and software components 312. In oneexample, the control device 300 may be a dedicated controller for themedia playback system 100. In another example, the control device 300may be a network device on which media playback system controllerapplication software may be installed, such as for example, an iPhone™,iPad™ or any other smart phone, tablet or network device (e.g., anetworked computer such as a PC or Mac™).

The processor 302 may be configured to perform functions relevant tofacilitating user access, control, and configuration of the mediaplayback system 100. The memory 304 may be data storage that can beloaded with one or more of the software components executable by theprocessor 302 to perform those functions. The memory 304 may also beconfigured to store the media playback system controller applicationsoftware and other data associated with the media playback system 100and the user.

In one example, the network interface 306 may be based on an industrystandard (e.g., infrared, radio, wired standards including IEEE 802.3,wireless standards including IEEE 802.11a, 802.11b, 802.11g, 802.11n,802.11ac, 802.15, 4G mobile communication standard, and so on). Thenetwork interface 306 may provide a means for the control device 300 tocommunicate with other devices in the media playback system 100. In oneexample, data and information (e.g., such as a state variable) may becommunicated between control device 300 and other devices via thenetwork interface 306. For instance, playback zone and zone groupconfigurations in the media playback system 100 may be received by thecontrol device 300 from a playback device or another network device, ortransmitted by the control device 300 to another playback device ornetwork device via the network interface 306. In some cases, the othernetwork device may be another control device.

Playback device control commands such as volume control and audioplayback control may also be communicated from the control device 300 toa playback device via the network interface 306. As suggested above,changes to configurations of the media playback system 100 may also beperformed by a user using the control device 300. The configurationchanges may include adding/removing one or more playback devices to/froma zone, adding/removing one or more zones to/from a zone group, forminga bonded or consolidated player, separating one or more playback devicesfrom a bonded or consolidated player, among others. Accordingly, thecontrol device 300 may sometimes be referred to as a controller, whetherthe control device 300 is a dedicated controller or a network device onwhich media playback system controller application software isinstalled.

Control device 300 may include microphone(s) 310. Microphone(s) 310 maybe arranged to detect sound in the environment of the control device300. Microphone(s) 310 may be any type of microphone now known or laterdeveloped such as a condenser microphone, electret condenser microphone,or a dynamic microphone. The microphone(s) may be sensitive to a portionof a frequency range. Two or more microphones 310 may be arranged tocapture location information of an audio source (e.g., voice, audiblesound) and/or to assist in filtering background noise.

The user interface 308 of the control device 300 may be configured tofacilitate user access and control of the media playback system 100, byproviding a controller interface such as the controller interface 400shown in FIG. 4. The controller interface 400 includes a playbackcontrol region 410, a playback zone region 420, a playback status region430, a playback queue region 440, and an audio content sources region450. The user interface 400 as shown is just one example of a userinterface that may be provided on a network device such as the controldevice 300 of FIG. 3 (and/or the control devices 126 and 128 of FIG. 1)and accessed by users to control a media playback system such as themedia playback system 100. Other user interfaces of varying formats,styles, and interactive sequences may alternatively be implemented onone or more network devices to provide comparable control access to amedia playback system.

The playback control region 410 may include selectable (e.g., by way oftouch or by using a cursor) icons to cause playback devices in aselected playback zone or zone group to play or pause, fast forward,rewind, skip to next, skip to previous, enter/exit shuffle mode,enter/exit repeat mode, enter/exit cross fade mode. The playback controlregion 410 may also include selectable icons to modify equalizationsettings, and playback volume, among other possibilities.

The playback zone region 420 may include representations of playbackzones within the media playback system 100. In some embodiments, thegraphical representations of playback zones may be selectable to bringup additional selectable icons to manage or configure the playback zonesin the media playback system, such as a creation of bonded zones,creation of zone groups, separation of zone groups, and renaming of zonegroups, among other possibilities.

For example, as shown, a “group” icon may be provided within each of thegraphical representations of playback zones. The “group” icon providedwithin a graphical representation of a particular zone may be selectableto bring up options to select one or more other zones in the mediaplayback system to be grouped with the particular zone. Once grouped,playback devices in the zones that have been grouped with the particularzone will be configured to play audio content in synchrony with theplayback device(s) in the particular zone. Analogously, a “group” iconmay be provided within a graphical representation of a zone group. Inthis case, the “group” icon may be selectable to bring up options todeselect one or more zones in the zone group to be removed from the zonegroup. Other interactions and implementations for grouping andungrouping zones via a user interface such as the user interface 400 arealso possible. The representations of playback zones in the playbackzone region 420 may be dynamically updated as playback zone or zonegroup configurations are modified.

The playback status region 430 may include graphical representations ofaudio content that is presently being played, previously played, orscheduled to play next in the selected playback zone or zone group. Theselected playback zone or zone group may be visually distinguished onthe user interface, such as within the playback zone region 420 and/orthe playback status region 430. The graphical representations mayinclude track title, artist name, album name, album year, track length,and other relevant information that may be useful for the user to knowwhen controlling the media playback system via the user interface 400.

The playback queue region 440 may include graphical representations ofaudio content in a playback queue associated with the selected playbackzone or zone group. In some embodiments, each playback zone or zonegroup may be associated with a playback queue containing informationcorresponding to zero or more audio items for playback by the playbackzone or zone group. For instance, each audio item in the playback queuemay comprise a uniform resource identifier (URI), a uniform resourcelocator (URL) or some other identifier that may be used by a playbackdevice in the playback zone or zone group to find and/or retrieve theaudio item from a local audio content source or a networked audiocontent source, possibly for playback by the playback device.

In one example, a playlist may be added to a playback queue, in whichcase information corresponding to each audio item in the playlist may beadded to the playback queue. In another example, audio items in aplayback queue may be saved as a playlist. In a further example, aplayback queue may be empty, or populated but “not in use” when theplayback zone or zone group is playing continuously streaming audiocontent, such as Internet radio that may continue to play untilotherwise stopped, rather than discrete audio items that have playbackdurations. In an alternative embodiment, a playback queue can includeInternet radio and/or other streaming audio content items and be “inuse” when the playback zone or zone group is playing those items. Otherexamples are also possible.

When playback zones or zone groups are “grouped” or “ungrouped,”playback queues associated with the affected playback zones or zonegroups may be cleared or re-associated. For example, if a first playbackzone including a first playback queue is grouped with a second playbackzone including a second playback queue, the established zone group mayhave an associated playback queue that is initially empty, that containsaudio items from the first playback queue (such as if the secondplayback zone was added to the first playback zone), that contains audioitems from the second playback queue (such as if the first playback zonewas added to the second playback zone), or a combination of audio itemsfrom both the first and second playback queues. Subsequently, if theestablished zone group is ungrouped, the resulting first playback zonemay be re-associated with the previous first playback queue, or beassociated with a new playback queue that is empty or contains audioitems from the playback queue associated with the established zone groupbefore the established zone group was ungrouped. Similarly, theresulting second playback zone may be re-associated with the previoussecond playback queue, or be associated with a new playback queue thatis empty, or contains audio items from the playback queue associatedwith the established zone group before the established zone group wasungrouped. Other examples are also possible.

Referring back to the user interface 400 of FIG. 4, the graphicalrepresentations of audio content in the playback queue region 440 mayinclude track titles, artist names, track lengths, and other relevantinformation associated with the audio content in the playback queue. Inone example, graphical representations of audio content may beselectable to bring up additional selectable icons to manage and/ormanipulate the playback queue and/or audio content represented in theplayback queue. For instance, a represented audio content may be removedfrom the playback queue, moved to a different position within theplayback queue, or selected to be played immediately, or after anycurrently playing audio content, among other possibilities. A playbackqueue associated with a playback zone or zone group may be stored in amemory on one or more playback devices in the playback zone or zonegroup, on a playback device that is not in the playback zone or zonegroup, and/or some other designated device.

The audio content sources region 450 may include graphicalrepresentations of selectable audio content sources from which audiocontent may be retrieved and played by the selected playback zone orzone group. Discussions pertaining to audio content sources may be foundin the following section.

d. Example Audio Content Sources

As indicated previously, one or more playback devices in a zone or zonegroup may be configured to retrieve for playback audio content (e.g.according to a corresponding URI or URL for the audio content) from avariety of available audio content sources. In one example, audiocontent may be retrieved by a playback device directly from acorresponding audio content source (e.g., a line-in connection). Inanother example, audio content may be provided to a playback device overa network via one or more other playback devices or network devices.

Example audio content sources may include a memory of one or moreplayback devices in a media playback system such as the media playbacksystem 100 of FIG. 1, local music libraries on one or more networkdevices (such as a control device, a network-enabled personal computer,or a networked-attached storage (NAS), for example), streaming audioservices providing audio content via the Internet (e.g., the cloud), oraudio sources connected to the media playback system via a line-in inputconnection on a playback device or network devise, among otherpossibilities.

In some embodiments, audio content sources may be regularly added orremoved from a media playback system such as the media playback system100 of FIG. 1. In one example, an indexing of audio items may beperformed whenever one or more audio content sources are added, removedor updated. Indexing of audio items may involve scanning foridentifiable audio items in all folders/directory shared over a networkaccessible by playback devices in the media playback system, andgenerating or updating an audio content database containing metadata(e.g., title, artist, album, track length, among others) and otherassociated information, such as a URI or URL for each identifiable audioitem found. Other examples for managing and maintaining audio contentsources may also be possible.

The above discussions relating to playback devices, controller devices,playback zone configurations, and media content sources provide onlysome examples of operating environments within which functions andmethods described below may be implemented. Other operating environmentsand configurations of media playback systems, playback devices, andnetwork devices not explicitly described herein may also be applicableand suitable for implementation of the functions and methods.

e. Example Plurality of Networked Devices

FIG. 5 shows an example plurality of devices 500 that may be configuredto provide an audio playback experience based on voice control. Onehaving ordinary skill in the art will appreciate that the devices shownin FIG. 5 are for illustrative purposes only, and variations includingdifferent and/or additional devices may be possible. As shown, theplurality of devices 500 includes computing devices 504, 506, and 508;network microphone devices (NMDs) 512, 514, and 516; playback devices(PBDs) 532, 534, 536, and 538; and a controller device (CR) 522.

Each of the plurality of devices 500 may be network-capable devices thatcan establish communication with one or more other devices in theplurality of devices according to one or more network protocols, such asNFC, Bluetooth, Ethernet, and IEEE 802.11, among other examples, overone or more types of networks, such as wide area networks (WAN), localarea networks (LAN), and personal area networks (PAN), among otherpossibilities.

As shown, the computing devices 504, 506, and 508 may be part of a cloudnetwork 502. The cloud network 502 may include additional computingdevices. In one example, the computing devices 504, 506, and 508 may bedifferent servers. In another example, two or more of the computingdevices 504, 506, and 508 may be modules of a single server.Analogously, each of the computing device 504, 506, and 508 may includeone or more modules or servers. For ease of illustration purposesherein, each of the computing devices 504, 506, and 508 may beconfigured to perform particular functions within the cloud network 502.For instance, computing device 508 may be a source of audio content fora streaming music service.

As shown, the computing device 504 may be configured to interface withNMDs 512, 514, and 516 via communication path 542. NMDs 512, 514, and516 may be components of one or more “Smart Home” systems. In one case,NMDs 512, 514, and 516 may be physically distributed throughout ahousehold, similar to the distribution of devices shown in FIG. 1. Inanother case, two or more of the NMDs 512, 514, and 516 may bephysically positioned within relative close proximity of one another.Communication path 542 may comprise one or more types of networks, suchas a WAN including the Internet, LAN, and/or PAN, among otherpossibilities.

In one example, one or more of the NMDs 512, 514, and 516 may be devicesconfigured primarily for audio detection. In another example, one ormore of the NMDs 512, 514, and 516 may be components of devices havingvarious primary utilities. For instance, as discussed above inconnection to FIGS. 2 and 3, one or more of NMDs 512, 514, and 516 maybe the microphone(s) 220 of playback device 200 or the microphone(s) 310of network device 300. Further, in some cases, one or more of NMDs 512,514, and 516 may be the playback device 200 or network device 300. In anexample, one or more of NMDs 512, 514, and/or 516 may include multiplemicrophones arranged in a microphone array.

As shown, the computing device 506 may be configured to interface withCR 522 and PBDs 532, 534, 536, and 538 via communication path 544. Inone example, CR 522 may be a network device such as the network device200 of FIG. 2. Accordingly, CR 522 may be configured to provide thecontroller interface 400 of FIG. 4. Similarly, PBDs 532, 534, 536, and538 may be playback devices such as the playback device 300 of FIG. 3.As such, PBDs 532, 534, 536, and 538 may be physically distributedthroughout a household as shown in FIG. 1. For illustration purposes,PBDs 536 and 538 may be part of a bonded zone 530, while PBDs 532 and534 may be part of their own respective zones. As described above, thePBDs 532, 534, 536, and 538 may be dynamically bonded, grouped,unbonded, and ungrouped. Communication path 544 may comprise one or moretypes of networks, such as a WAN including the Internet, LAN, and/orPAN, among other possibilities.

In one example, as with NMDs 512, 514, and 516, CR522 and PBDs 532, 534,536, and 538 may also be components of one or more “Smart Home” systems.In one case, PBDs 532, 534, 536, and 538 may be distributed throughoutthe same household as the NMDs 512, 514, and 516. Further, as suggestedabove, one or more of PBDs 532, 534, 536, and 538 may be one or more ofNMDs 512, 514, and 516.

The NMDs 512, 514, and 516 may be part of a local area network, and thecommunication path 542 may include an access point that links the localarea network of the NMDs 512, 514, and 516 to the computing device 504over a WAN (communication path not shown). Likewise, each of the NMDs512, 514, and 516 may communicate with each other via such an accesspoint.

Similarly, CR 522 and PBDs 532, 534, 536, and 538 may be part of a localarea network and/or a local playback network as discussed in previoussections, and the communication path 544 may include an access pointthat links the local area network and/or local playback network of CR522 and PBDs 532, 534, 536, and 538 to the computing device 506 over aWAN. As such, each of the CR 522 and PBDs 532, 534, 536, and 538 mayalso communicate with each over such an access point.

In one example, a single access point may include communication paths542 and 544. In an example, each of the NMDs 512, 514, and 516, CR 522,and PBDs 532, 534, 536, and 538 may access the cloud network 502 via thesame access point for a household.

As shown in FIG. 5, each of the NMDs 512, 514, and 516, CR 522, and PBDs532, 534, 536, and 538 may also directly communicate with one or more ofthe other devices via communication means 546. Communication means 546as described herein may involve one or more forms of communicationbetween the devices, according to one or more network protocols, overone or more types of networks, and/or may involve communication via oneor more other network devices. For instance, communication means 546 mayinclude one or more of for example, Bluetooth™ (IEEE 802.15), NFC,Wireless direct, and/or Proprietary wireless, among other possibilities.

In one example, CR 522 may communicate with NMD 512 over Bluetooth™, andcommunicate with PBD 534 over another local area network. In anotherexample, NMD 514 may communicate with CR 522 over another local areanetwork, and communicate with PBD 536 over Bluetooth. In a furtherexample, each of the PBDs 532, 534, 536, and 538 may communicate witheach other according to a spanning tree protocol over a local playbacknetwork, while each communicating with CR 522 over a local area network,different from the local playback network. Other examples are alsopossible.

In some cases, communication means between the NMDs 512, 514, and 516,CR 522, and PBDs 532, 534, 536, and 538 may change depending on types ofcommunication between the devices, network conditions, and/or latencydemands. For instance, communication means 546 may be used when NMD 516is first introduced to the household with the PBDs 532, 534, 536, and538. In one case, the NMD 516 may transmit identification informationcorresponding to the NMD 516 to PBD 538 via NFC, and PBD 538 may inresponse, transmit local area network information to NMD 516 via NFC (orsome other form of communication). However, once NMD 516 has beenconfigured within the household, communication means between NMD 516 andPBD 538 may change. For instance, NMD 516 may subsequently communicatewith PBD 538 via communication path 542, the cloud network 502, andcommunication path 544. In another example, the NMDs and PBDs may nevercommunicate via local communications means 546. In a further example,the NMDs and PBDs may communicate primarily via local communicationsmeans 546. Other examples are also possible.

In an illustrative example, NMDs 512, 514, and 516 may be configured toreceive voice inputs to control PBDs 532, 534, 536, and 538. Theavailable control commands may include any media playback systemcontrols previously discussed, such as playback volume control, playbacktransport controls, music source selection, and grouping, among otherpossibilities. In one instance, NMD 512 may receive a voice input tocontrol one or more of the PBDs 532, 534, 536, and 538. In response toreceiving the voice input, NMD 512 may transmit via communication path542, the voice input to computing device 504 for processing. In oneexample, the computing device 504 may convert the voice input to anequivalent text command, and parse the text command to identify acommand. Computing device 504 may then subsequently transmit the textcommand to the computing device 506. In another example, the computingdevice 504 may convert the voice input to an equivalent text command,and then subsequently transmit the text command to the computing device506. The computing device 506 may then parse the text command toidentify one or more playback commands.

For instance, if the text command is “Play ‘Track 1’ by ‘Artist 1’ from‘Streaming Service 1’ in ‘Zone 1’,” The computing device 506 mayidentify (i) a URL for “Track 1” by “Artist 1” available from “StreamingService 1,” and (ii) at least one playback device in “Zone 1.” In thisexample, the URL for “Track 1” by “Artist 1” from “Streaming Service 1”may be a URL pointing to computing device 508, and “Zone 1” may be thebonded zone 530. As such, upon identifying the URL and one or both ofPBDs 536 and 538, the computing device 506 may transmit viacommunication path 544 to one or both of PBDs 536 and 538, theidentified URL for playback. One or both of PBDs 536 and 538 mayresponsively retrieve audio content from the computing device 508according to the received URL, and begin playing “Track 1” by “Artist 1”from “Streaming Service 1.”

One having ordinary skill in the art will appreciate that the above isjust one illustrative example, and that other implementations are alsopossible. In one case, operations performed by one or more of theplurality of devices 500, as described above, may be performed by one ormore other devices in the plurality of device 500. For instance, theconversion from voice input to the text command may be alternatively,partially, or wholly performed by another device or devices, such as NMD512, computing device 506, PBD 536, and/or PBD 538. Analogously, theidentification of the URL may be alternatively, partially, or whollyperformed by another device or devices, such as NMD 512, computingdevice 504, PBD 536, and/or PBD 538.

f. Example Network Microphone Device

FIG. 6 shows a function block diagram of an example network microphonedevice 600 that may be configured to be one or more of NMDs 512, 514,and 516 of FIG. 5. As shown, the network microphone device 600 includesa processor 602, memory 604, a microphone array 606, a network interface608, a user interface 610, software components 612, and speaker(s) 614.One having ordinary skill in the art will appreciate that other networkmicrophone device configurations and arrangements are also possible. Forinstance, network microphone devices may alternatively exclude thespeaker(s) 614 or have a single microphone instead of microphone array606.

The processor 602 may include one or more processors and/or controllers,which may take the form of a general or special-purpose processor orcontroller. For instance, the processing unit 602 may includemicroprocessors, microcontrollers, application-specific integratedcircuits, digital signal processors, and the like. The memory 604 may bedata storage that can be loaded with one or more of the softwarecomponents executable by the processor 602 to perform those functions.Accordingly, memory 604 may comprise one or more non-transitorycomputer-readable storage mediums, examples of which may includevolatile storage mediums such as random access memory, registers, cache,etc. and non-volatile storage mediums such as read-only memory, ahard-disk drive, a solid-state drive, flash memory, and/or anoptical-storage device, among other possibilities.

The microphone array 606 may be a plurality of microphones arranged todetect sound in the environment of the network microphone device 600.Microphone array 606 may include any type of microphone now known orlater developed such as a condenser microphone, electret condensermicrophone, or a dynamic microphone, among other possibilities. In oneexample, the microphone array may be arranged to detect audio from oneor more directions relative to the network microphone device. Themicrophone array 606 may be sensitive to a portion of a frequency range.In one example, a first subset of the microphone array 606 may besensitive to a first frequency range, while a second subset of themicrophone array may be sensitive to a second frequency range. Themicrophone array 606 may further be arranged to capture locationinformation of an audio source (e.g., voice, audible sound) and/or toassist in filtering background noise. Notably, in some embodiments themicrophone array may consist of only a single microphone, rather than aplurality of microphones.

The network interface 608 may be configured to facilitate wirelessand/or wired communication between various network devices, such as, inreference to FIG. 5, CR 522, PBDs 532-538, computing device 504-508 incloud network 502, and other network microphone devices, among otherpossibilities. As such, network interface 608 may take any suitable formfor carrying out these functions, examples of which may include anEthernet interface, a serial bus interface (e.g., FireWire, USB 2.0,etc.), a chipset and antenna adapted to facilitate wirelesscommunication, and/or any other interface that provides for wired and/orwireless communication. In one example, the network interface 608 may bebased on an industry standard (e.g., infrared, radio, wired standardsincluding IEEE 802.3, wireless standards including IEEE 802.11a,802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G mobile communicationstandard, and so on).

The user interface 610 of the network microphone device 600 may beconfigured to facilitate user interactions with the network microphonedevice. In one example, the user interface 608 may include one or moreof physical buttons, graphical interfaces provided on touch sensitivescreen(s) and/or surface(s), among other possibilities, for a user todirectly provide input to the network microphone device 600. The userinterface 610 may further include one or more of lights and thespeaker(s) 614 to provide visual and/or audio feedback to a user. In oneexample, the network microphone device 600 may further be configured toplayback audio content via the speaker(s) 614. In this case, the NMD 600may also include the functions and features associated with the playbackdevice 200.

III. Example Systems and Methods for Bass Enhancement

An audio playback device may play audio content at different volumesettings. An audio signal may be input into the audio playback deviceand the audio playback device may output audio based on the audiosignal.

A bass response is low frequency content of an audio signal played bythe audio playback device. The low frequency content may be typically inthe range of 0 to 100 Hz. The bass response may be boosted by adjustingan audio signal to be played by the audio playback device. Theadjustment may take the form of increasing a gain of the low frequencycontent of the audio signal played by the audio playback device to boostthe bass response of the playback device. Typically, the bass may beboosted at the different volumes to correct for a natural response (EQ)of a speaker and/or acoustics of room and/or the content.

The audio playback device may have a certain operational limit (e.g.,mechanical, electrical, thermal) beyond which audio quality maydeteriorate and/or components of the playback device might becomedamaged. The operational limit might include maximum movement (alsoreferred to herein as an excursion) of a surround, spider, voice coiland cone from a rest position when an audio signal is applied to atransducer or passive radiator. For example, the operational limit ofthe cone may be that it cannot flex more than 0.5 cm before beingdamaged during playback of audio. Additionally, or alternatively, theoperational limit might include voltage limits, current limits, powerlimits, temperature limits, and any other physical parameters associatedwith the audio playback device when playing audio. For example, thecurrent shall not exceed 800 mA and the voltage shall not exceed 10volts, otherwise the playback device may be damaged. Further, currentmay be controlled based on impedance in the playback device. Forexample, a maximum amount of current may flow in a circuit of the audioplayback device when the circuit has a minimum impedance. Theoperational limit may specify a limiting voltage (based on knowledge ofthe impedance) at the minimum impedance to control current flow in theplayback device when playing audio. In some instances, the minimumimpedance may be determined based on a frequency of the audio signal.

Additionally, or alternatively the operational limit may be a maximumpower of the audio signal input into the playback device. The power ofthe audio signal may impact a temperature of certain components in thetransducer such as a voice coil, wire to the voice coil, glue thatconnects the wire to the voiced coil, and/or wire insulation.Specifically, as power of the audio signal increases, the temperature ofthe components in the playback device may increase. It follows that ifthe temperature of the components gets too high, the components of theplayback device can be damaged. In this regard, defining an operationallimit in terms of power may prevent such damage.

With bass boosting, the playback device may approach the operationallimit before a volume setting approaches a highest volume setting. Tomaintain operation of the playback device within the operational limit,the audio signal to be played by the playback device can be limited toprevent the playback device from exceeding the operational limit as thevolume continues to increase.

The conventional limiting methods may take a variety of forms. However,a problem with the conventional limiting methods is that it produces achoppy listening experience. For example, in certain base heavy content(e.g., acoustic content with a lot of bass such as a kick drum), theloudness of the kick drum may be inconsistent as filter characteristics,e.g., a threshold frequency, Q, and/or gain, of a digital filter appliedto the audio signal are dynamically adjusted as a function of time, theaudio signal, and/or a previous threshold frequency, Q, and/or gain.Quieter beats of the kickdrum may be limited less whereas louder beatsof the kick drum (which are expected to exceed the transducer's physicallimits) may be limited more. The difference in limiting may result inthe bass appearing to unnaturally cut in and out.

Embodiments described herein are directed to a limiter for bass boostingwhich reduces the choppiness associated with conventional schemes forlimiting the audio signal to be played by an audio playback device. Theaudio playback device may operate within an operational limit withoutneeding to dynamically adjust filter characteristics that wouldotherwise produce the undesirable choppy audio. In this regard, anydistortion may not be perceivable or minimally perceivable. Also,stresses on the audio playback device, e.g., transducer stress, may notincrease.

In one embodiment, functions associated with the limiter may beimplemented on the audio playback device. For example, the processor 202of the audio playback device may perform functions associated with thedescribed limiter. In another embodiment, functions associated with thelimiter may be implemented on a controller device 300. In yet anotherembodiment, functions associated with the limiter may be implemented ona computing device 506-508 in the cloud network 502. For example, theaudio playback device may send an audio signal to be played back to thecomputing device 506-508. Based on the audio signal and knowledge of anarrangement of the playback device, e.g., type of playback device,arrangement of transducers in the playback device, capabilities of theplayback device, etc., the computing device may limit the audio signaland provide this signal back to the audio playback device for playback.In another embodiment, functions associated with the limiter may beimplemented partially on one or more of the computing device, the audioplayback device, and the controller in one or more combinations. Othervariations are also possible.

FIG. 7 shows an example arrangement 700 associated with the describedlimiting. The arrangement 700 may include an input 702, an output 704, alimiter 706, and an audio output pipeline 708.

The input 702 may be an audio signal. In one example, the audio signalmay be a digital audio signal such as a packetized or non-packetizedstream of audio from a music service or television, a digital audiofile, an audio signal generated by the audio playback device itself or adevice connected to the audio playback device. For example, the packetmay comprise 128 bits of audio data. The limiter 706 may process one ormore packets of audio data in a time domain and/or frequency domain. Asampling rate of audio data in the one or more packets and number ofsamples may define a frequency range of the audio processed by thelimiter 706.

In another example, the audio signal may be analog signal input from anauxiliary connection or a digital signal input from a USB connection.The audio signal may comprise frequency content that may range from 0 Hzto 22,050 Hz or some subset of this frequency range.

The limiter 706 may process the audio signal to generally prevent theaudio playback device from exceeding an operational limit. At the sametime, the processing may not introduce choppiness in the audio playbackassociated with limiting bass. The limiter 706 may process one or morepackets of audio data at a time. The number of packets processed by thelimiter 700 at a time may depend on a frequency of the audio contentdefined by the audio data.

The output 704 of the limiter 706 may be a mixed audio signal. Forexample, the mixed audio signal may be one or more packets of audio datacomposed of the audio signal (e.g., dry audio signal) and a processedversion of the dry signal (e.g., wet audio signal). The limiter 706 mayprevent the audio playback device from generally exceeding anoperational limit of the playback device or more specifically exceedingan operational limit of the audio playback pipeline 708 of the playbackdevice. In this regard, the mixed audio signal may be input to adownstream processing module 708 to cause the playback device to playaudio.

The downstream processing module 708 may be part of the audio processingpipeline include components and/or processing associated with playbackof audio by audio playback device 200. For example, the downstreamprocessing module 708 may include one or more speakers 212 and audioprocessing components 208. The speakers 212 may be an “activeloudspeaker” (or main driver) or a passive radiator (also known as a“drone cone”). The active loudspeaker may be a conventional driver, andthe passive radiator is typically the same or similar, but without avoice coil and magnet assembly. The passive radiator can be a suspendedcone, not attached to a voice coil or wired to electrical circuitry andnot connected to a power amplifier. The audio processing components 208may include crossover circuits and electronic filter circuitry. Thecrossover circuitry may be electronic filter circuitry used in a rangeof audio applications, to split up an audio signal into two or morefrequency ranges, so that the signals can be sent to drivers or tweetersthat are designed for different frequency ranges. Additionally, oralternatively, the downstream processing module 708 may be electronicfilter circuitry which may limit audio signals, e.g., by clipping theaudio signals. Still additionally, or alternatively, the downstreamprocessing module 708 may include an audio amplifier 210 to performequalization. The downstream processing module 708 may include othercomponents such as a digital to analog converter (DAC) as well.

In this regard, the operational limit of the playback device may includeelectrical limits, mechanical limits, thermal limits, and/or otherphysical limits associated with functionality of the downstreamprocessing module 708. The mechanical limits may be the excursionlimits, e.g., maximum movement, of the passive radiator or the activeloudspeaker. The electrical limits may include voltage and/or currentlimits associated with the circuitry in the audio playback pipeline 708such as analog or digital filters. The thermal limits may includetemperature limit associated with the circuitry. Other operationallimits may exist as well.

FIG. 8 shows an example of a dynamic sidechain filter (DSF) 800. The DSFapplies filtering as a “side chain” to the dry audio signal associatedwith the audio signal pipeline. The limiter 706 of FIG. 7 may take theform of one instance of the DSF 800 or two or more instances of the DSF800. The two or more DSFs may be cascaded together such that an outputof one DSF is an input to another DSF. The DSF 800 may have an input 802for receiving a dry audio signal and an output 804 for outputting asignal (e.g., mixed signal) which comprises a weighted mix of the dryaudio signal and a wet audio signal which is described in more detailbelow. The mixing of the dry and wet audio signal at different levelsover time as described below makes the DSF “dynamic.”

For the implementation and other processes and methods disclosed herein,the arrangement shows functionality and operation of one possibleimplementation of some embodiments. In this regard, each block mayrepresent a module, a segment, or a portion of program code, whichincludes one or more instructions executable by a processor forimplementing specific logical functions or steps in the process. Theprogram code may be stored on any type of computer readable medium, forexample, such as a storage device including a disk or hard drive. Thecomputer readable medium may include non-transitory computer readablemedium, for example, such as tangible, non-transitory computer-readablemedia that stores data for short periods of time like register memory,processor cache and Random Access Memory (RAM). The computer readablemedium may also include non-transitory media, such as secondary orpersistent long term storage, like read only memory (ROM), optical ormagnetic disks, compact-disc read only memory (CD-ROM), for example. Thecomputer readable media may also be any other volatile or non-volatilestorage systems. The computer readable medium may be considered acomputer readable storage medium, for example, or a tangible storagedevice. In addition, for the implementation and other processes andmethods disclosed herein, each block in FIG. 8 may represent circuitrythat is wired to perform the specific logical functions in the process.

The dry audio signal may be an audio signal received directly from anaudio source. Additionally, or alternatively, the dry audio signal maybe an audio signal processed by the audio playback device. In oneexample, the audio signal may be processed by an equalizer prior tobeing input to the DSF 800. In another example, the audio signal may bemixed with one or more audio sources prior to being input to the DSF800. In yet another example, a uniform gain may be added to the audiosignal to account for a volume setting prior to being input to the DSF800 but before being amplified for output by a transducer. In the caseof a cascade of DSFs 800, the dry audio signal may be an output of aprior DSF 800 in the cascade.

The wet audio signal may be the dry audio signal filtered by a filter814. The filter 814 may have a gain reduction frequency response. Forexample, the frequency response may be dominated by a range offrequencies with less than unity gain (also known as a cutting region)and a remaining range of frequencies at or below unity. In some cases,the filter 814 may define a maximum gain reduction of the limiter. Thefilter 814 applied to the dry signal may take a variety of forms. Forexample, the filter 814 may be a fixed equalization filter (e.g.,characteristics do not change over time) applied to the audio signal tofilter the audio signal in a manner which is acoustically pleasing to alistener while conforming to the operational limit of the audio playbackpipeline. The filter 814 may take the form of a finite impulse responsefilter (FIR), warped FIR, infinite impulse response filter (IIR), amongother examples. The filter 814 may allow for the operational limit to bereached most readily in the low frequencies as volume increases. Then,as the volume continues to increase, a spectral balance of gain maychange such that gain is increased for higher frequencies resulting in aperceived increase in loudness without the transducer exceedingoperational limits.

FIG. 9 shows one example of this filter 814. The filter 814 may be a lowband reduction filter which is applied to low frequency bands of the dryaudio signal. For example, the low-band reduction filter may cover the 0to 45 Hz range and attenuate the dry audio signal in the 0 to 10 Hzrange which is typically not perceptible by a listener but may result ingreatest reduction in excursions. In addition, the low-band reductionfilter may have a frequency boost above a frequency threshold at whichphysical limits are less of a concern. In the example filter 814, thefrequency threshold is 100 Hz. This frequency boost may provide tonalbalance, adding upper-bass to replace reduced lower bass to balanceaudio for a listener across an entire frequency range. The low-bandreduction filter may take a variety of other forms, including beingfurther adjusted to fit physical and/or operational characteristics ofthe audio playback device on which its applied.

FIG. 10 shows a second example of this filter 814. The filter 814 may bea mid-band reduction filter. For example, the mid band reduction filtermay cover a 0 to 100 Hz range and attenuate the dry audio signal in a 10to 100 Hz range. The attenuation may be perceptible to a listener inthis frequency range. The mid band filter may reduce excursions lessthan the low band filter. The mid band filter may have a frequency boostabove a frequency threshold at which physical limits are less of aconcern. The frequency boost may compensate for the attenuation. In theexample filter, the frequency threshold is also 100 Hz. Like thelow-band reduction filter, the mid-band reduction filter may take avariety of other forms, including being further adjusted to fit physicaland/or operational characteristics of the audio playback device on whichits applied.

FIG. 11 shows a third example of this filter 814. The filter 814 may bea wide-shelf reduction filter. The wide band reduction filter mayattenuate in a frequency range which overlaps the low-band and mid-bandreduction filters to bring down amplitudes of a wide range offrequencies from 0 to 100 Hz. Like the mid band reduction filter, theattenuation may be perceptible to a listener. The wide-shelf reductionfilter may not have any frequency boost. Like the low-band filter andthe mid-band reduction filter, the wide shelf reduction filter may takea variety of other forms, including being further adjusted to fitphysical and/or operational characteristics of the audio playback deviceon which its applied.

The DSF 800 may store in memory and/or request and/or receive from anetwork device an indication of operational limit of the audio playbackdevice. The indication may take the form of a threshold 806. Thethreshold 806 may be indicative of a maximum signal output by the DSF800. The threshold 806 may be the same across all frequencies of the drysignal input into the DSF 800.

Alternatively, the threshold 806 may be a performance ceiling response.The performance ceiling response may define a maximum signal output bythe DSF 800 as a function of frequency so that the audio playbackpipeline does not exceed the operational limit (e.g., conforming toexcursion, voltage, current, or power limits). In one example, theperformance ceiling response may define an operational limit in terms ofan excursion limit of the audio playback device. In a second example,the performance ceiling response may define an operational limit interms of a voltage limit of the audio playback device. In a thirdexample, the performance ceiling response may define an operation limitin terms of a current limit of the audio playback device. In a fourthexample, the performance ceiling response may define an operationallimit in terms of a power limit of the audio playback device. In a fifthexample, the performance ceiling response may define an operation limitin terms of an aggregation of one or more of the performance ceilingresponse associated with the excursion limit, voltage limit, currentlimit, power limit etc. For example, the performance ceiling responsemay define a minimum or maximum operation limit of one or more of theexcursion limit, voltage limit, current limit, or power limit for acertain frequency or range of frequencies. Other variations are alsopossible.

The performance ceiling response may be determined during a testing ofthe audio playback pipeline and then made available to the DSF 800. Thetesting may be performed by a manufacturer of the audio playback device.For example, audio signals may be applied at different amplitudes anddifferent frequencies to the audio playback pipeline to determine whenan operational limit of the audio playback pipeline is reached, e.g.,the playback device is damaged or about to be damaged. In the case ofthe playback device having multiple woofers, it is assumed that eachwoofer has a same volume displacement. The multiple woofers may betreated as one woofer with each woofer having a 1/n share of the volumedisplacement where n is the number of woofers for purposes ofdetermining an operational limit. In the case of the playback devicehaving multiple transducers, it is assumed that bass content isconsolidated in a bass channel for purposes of determining anoperational limit.

Alternatively, the performance ceiling response may be determined basedon analyzing one or more physical models that describe operation of theaudio playback device, e.g., audio playback pipeline, as a lumpedcircuit/mechanical system. The analysis may indicate what audio signalsmay be applied at different amplitudes and different frequencies to theaudio playback pipeline to stay within an operational limit of the audioplayback pipeline. The performance ceiling response may be determined inother ways as well.

The representation of the threshold 806 may be defined mathematically ornumerically. The mathematical representation may take the form of one ormore biquads, e.g., amplitude vs frequency representations, orequalizations. The numerical representation may take the form of asingle threshold value, a plurality of threshold value, or one or moretables of values of amplitudes and corresponding frequencies.

An excess metric may be calculated to determine how much of the wetsignal to mix with the dry signal so that the playback device operateswithin an operational limit. The excess metric may be an amplitudeexcess over the threshold 806 at every frequency or frequency band to beconsidered. The excess metric may be determined over a full spectrum orwithin a given frequency band such as 0 to 45 Hz when a low band filteris applied or 45-100 Hz when a mid-band filter is applied.Alternatively, the excess metric may be determined in a time domain,e.g., amplitude excess over the threshold 806 over a period of time.

Referring back to FIG. 8, the dry detector 808 may specifically comparean envelope of the dry signal with the threshold 806 at dry detector808. To facilitate this process, the dry detector 808 may comprise apeak detector. In general, the peak detector may receive an audio signalas input and produce an envelope of the audio signal as output. The peakdetector may compute an absolute value of the audio signal. The absolutevalue may be an absolute value of a sample of the audio signal.Alternatively, the absolute value may be an absolute value of a sum ofsamples of the audio signal, e.g., in one or more audio packets. Thepeak detector may set an envelope value to the absolute value if theabsolute value exceeds a current envelope value and otherwise, decay thecurrent envelope value. The decay may be represented by:

new envelope value=current envelope value*k  Eq1:

where k is a value less than one that causes the envelope to decay to asuitably small fraction within a few tens of milliseconds. Typicalenvelope detector parameters may place the decay rate to about 50 ms toreach 5 percent of the original value. As a result, the peak detectorreconstructs the signal envelope by “bridging exponentially” acrossvalleys in the audio signal.

The output of the comparison performed by the dry detection 808 may be amathematical representation, e.g., biquad, or numerical representationof the amplitude of the envelope of the dry audio signal in excess ofthe threshold 806 to yield a dry excess. The dry excess may be adifference between the envelope of the dry audio signal and thethreshold 806. The difference may be at specific time, a specificfrequency, or over a range of time (e.g., over one or more packets) or arange of frequencies (in which case the differences may be integrated todefine the dry excess). The dry excess may be represented as a positivequantity if the amplitude of the envelope of the dry audio signal isgreater than the amplitude of the threshold 806. Conversely, the dryexcess may be represented as a negative quantity if the envelope of thedry audio signal is less than the threshold.

FIG. 12 shows an example of the comparison of a performance ceilingresponse 1202 associated with an audio playback pipeline with anenvelope of the dry audio signal 1204. A positive dry excess is shown asthe area 1206 of the envelope of the dry audio signal 1204 which exceedsthe performance ceiling response 1202. A negative dry excess is shown asthe area 1208 of the envelope of the dry audio signal 1204 which isbelow the performance ceiling response 1202. The comparison of theenvelope of the dry audio signal may be performed across an entirespectrum or at a given frequency band such as a low-band from 0 to 45 Hzor a mid-band from 45 to 100 Hz. The dry excess may be a maximum valuebetween the performance ceiling response 1202 and the envelope of thedry audio signal 1204 within the frequency band within the frequencyband or an integration of an area between the performance ceilingresponse 1202 and the envelope of the dry audio signal 1204 within thefrequency band.

In some examples, the dry detector 808 may compare the envelope of thedry audio signal 1203 (e.g., dry envelope) to one or more differentperformance ceiling responses associated with different operationallimits, e.g., excursion limits, voltage limits, current limits, etc. Thedry detector 808 may output a maximum dry excess based on thesecomparisons. Other variations are also possible.

Similarly, the envelope of the wet signal may be compared to thethreshold 806 at wet detector 810. An output of this comparison may be amathematical representation or numerical representation of the envelopeof the wet signal (e.g., wet envelope) in excess of the threshold 806 toyield a wet excess. In some examples, the threshold 806 associated withwet detector 810 and dry detector 808 may be identical to facilitatecomparisons based on respective outputs by 808, 810, but they could bedifferent. Further, the threshold 806 associated with wet detector 810and dry detector 808 could be implemented as a single module whichreceives a dry and wet signal rather than separate modules as shown inFIG. 8.

In some examples, the wet detector 810 may compare the wet signal itselfto the threshold 806 to determine the wet excess. In some examples, thedry detector 808 may compare the dry signal itself to the threshold 806to determine the dry excess. This may be instead of comparing to theenvelope of a wet/dry signal or in addition to comparing to the envelopeof the wet/dry signal.

The DSF 800 may have a controller 816 (not to be confused with thecontroller device 300). The controller 816 may be hardware, software, ora combination of hardware and software to receive one or more of the wetexcess, dry excess, wet envelope, dry envelope, and threshold 806 anddetermine how much the dry and wet signals are to be mixed at 812 tokeep the playback device below an applicable operational limit whileaffecting the dry signal as little as possible. The amount of mixing maybe based on a gain applied to the dry signal and a gain applied to thewet signal. For example, a proportion of 100% wet may indicate that thesignal output by the DSF 800 is only the wet signal and a proportion is0% wet may indicate that the signal output by the DSF 800 is only thedry signal. Any proportions in between may result in a blend of the wetsignal and dry signal where the percentage of wet signal and dry signalequals 100%. Further, the filter 814 which is applied may be chosen sothat there is a smooth progression between a 100% dry signal,intermediate choices of the wet signal and dry signal blend, and a 100%wet signal. The wide-shelf reduction filter is one example of such afilter which achieves this smooth progression.

In summary, the dry audio signal and the wet audio signal may generallybe mixed in proportion to respective gains. Mathematically, this may berepresented as:

Mixed Signal=dry signal*gain of dry signal+wet signal*gain of wetsignal  Eq 2:

where gain of dry signal+gain of wet signal=1. The gains may bedetermined at a frequency or a range of frequencies.

In one example, if both the wet envelope and the dry envelope are belowthe threshold 806 (e.g., wet and dry excess is negative) for a givenfrequency or range of frequencies, then the gain applied to the drysignal may be one and the gain applied to the wet signal may be zero.This indicates that the dry signal itself will not cause the audioplayback device, e.g., audio playback pipeline, to exceed theoperational limit. Further, the wet signal, although not relevant to theoutput of the DSF 800, may be equal to or lower than the dry signalbecause the filter 814 applied to the dry signal generally has afrequency response with a gain at or below unity.

In a second example, if the wet envelope is below the threshold 806(e.g., wet excess is negative) but the dry envelope is above thethreshold for a frequency or range of frequencies, then the gain appliedto the dry signal may be computed as:

dryGain=(wetEnvelope−threshold)/(wetEnvelope−dryEnvelope)  Eq. 3:

where the wetEnvelope may be a representation of the wet envelope andthe dryEnvelope may be a representation of the dry envelope. The dryGainmay be a representation of the gain to apply to the dry signal for agiven frequency or range of frequencies to reduce an amplitude of thedry signal. The representation may be mathematical, e.g., a biquad, ornumerical such as a table of values. WetGain may be computed as1−dryGain.

Intuitively, the DSF 800 may have identified that the envelope of thedry signal (e.g., dry envelope) is above the threshold 806, and theenvelope of the wet signal (e.g., wet envelope) is below the threshold806. In this case, the dry excess is a positive value, the wet excess isnot greater than zero, and correction of the dry excess is possible.Since the DSF 800 may strive to produce an output that does not exceedthe threshold, a wet/dry proportion should be chosen that causes theoutput to be as high as it can be, while remaining below the threshold806.

The equation dryGain=(wetEnvelope−threshold)/(wetEnvelope−dryEnvelope)which may achieve this result is derived as follows:

The wet gain and dry gain may be complements of each other adding up toone:

dryGain=1−wetGain  Eq 4:

As a result, the output of the limiter may be represented as:

Output=dryGain*drySignal+wetGain*wetSignal  Eq 5:

where the output is composed of the sum of wet signal and dry signalafter applying respective gains.

Next, substitute eq4 into eq5:

Output=dryGain*drySignal+(1−dryGain)*wetSignal  Eq 6:

Because the envelopes of the wet and dry signals may be a suitable upperbound to what values they may reach, the envelope of the output of theDSF 800 may not exceed the sum of the envelopes of the inputs (the drysignal and the wet signals combined in some proportion to bedetermined).

Eq 6 may be rewritten in terms of the envelopes for the wet and drysignals. Given that the output envelope can reach the threshold:

threshold=dryGain*dryEnvelope+(1−dryGain)*wetEnvelope  Eq 7:

and dryGain is solved for:

0=dryGain*dryEnvelope+wetEnvelope−dryGain*wetEnvelope−threshold

0=dryGain(dryEnvelope−wetEnvelope)+wet−threshold

dryGain(wetEnvelope−dryEnvelope)=wet−threshold

dryGain=(wetEnvelope−threshold)/(wetEnvelope−dryEnvelope)  Eq8:

In a third example, both the wet envelope and the dry envelope may beabove the threshold 806 for a given frequency or range of frequencies.

When both wet envelope and dry envelope are over the threshold 806 itcould be for various reasons: For example, a frequency response of thedry signal coming in may be well within a frequency range that thefilter 814 could reduce it. Alternatively, the frequency response of thedry signal may not be quite in that frequency range, and thereforeattempting a correction would apply unnecessary response changes to thedry signal that still results in the output exceeding the threshold 806.

One metric to judge whether the dry signal may be reduced by using thewet signal is a ratio between the wet and dry signal envelopes (theratio of the two peak detectors). A ratio may be chosen above which thedry signal is out of the range of viable gain reduction, and below whichwe consider the dry signal to be within the range of viable gainreduction. For example, a signal causing the wet/dry ratio to be 0.5would have a spectrum that results in a total peak output reduction of 6dB. It may be a design choice to decide whether to apply gain correctionbased on the wet/dry ratio.

A MWGR may be a minimum worthwhile gain reduction (MWGR) whichfacilitates making this decision. The DSF 800 may operate in that whenboth wet and dry envelopes exceed the threshold 806 and the wet/dryratio remains under the MWGR, the wet and dry mix output by the DSF 800may move toward full wet as an amplitude of the dry signal inputincreases (as long as the wet peak value remains under the threshold806) but once the wet peak value reaches the threshold 806 the wet drygain blend would in turn reach 100% wet (dryGain=0). And if the gain ofthe dry signal is increased such that now both the wet peak value andthe dry peak value have exceeded the threshold 806, then in turn thewet/dry blend would effectively remain clipped at 100% wet.

On the other hand, the wet/dry ratio may exceed the value of MWGR. Forexample, the wet/dry peak detector ratio may be 1. This is the case fora dry signal whose spectral content is completely outside the rangewhere the filter 814 can have any effect. In this scenario, the targetfor wet/dry gain should be 0% wet (dryGain=1) as the dry signal cannotbe reduced appreciably.

For all ratios in between, the output of the DSF 800 may be calculatedas a linear interpolation between a 100% wet signal and a 100% drysignal. On one extreme, the ratio is exactly at the MWGR, so a full wetgain is set as the target. On the other extreme the ratio is at 1, so afull dry gain is set at the target. The equation which would cause thedry gain to continuously change from full wet (dryGain=0) to full dry(dryGain=1) is:

dryGain=(wetEnvelope−dryEnvelope*MWGR)/(dryEnvelope−threshold)  Eq: 9:

The dryGain may be a representation of the gain to apply to the drysignal for a given frequency or range of frequencies. Further, when thedryEnvelope equals the threshold, then the frequency limiter may outputthe dry signal. Still further, if the dryGain is greater than one, thedryGain is limited to one. The wetEnvelope and dryEnvelope may be arepresentation of the envelope of the wet signal and dry signal,respectively. Again, the representation may be mathematical, e.g., abiquad, or numerical such as a table of values. The dry gain may beconstrained between 0 and 1. Further, the wetGain may be computed as1−dryGain.

In summary, the output of the DSF 800 may be based on a mix of the drysignal and the wet signal, where the dry and wet signal may be eachweighted from 0 to 1 and the sum of the weightings equals 1. This outputmay take the form of digital data such as one or more packets of audiosamples or a table of numerical values, or an analog signal.

To illustrate, consider that a dry signal may be 10 dB above performanceceiling response in the form of an excursion threshold and a wet signalmay be 2 dB (avg) below the excursion threshold in a frequency band suchas 30-50 Hz. This means that a dry signal which is 10 dB above theexcursion threshold may need to be reduced by 8 dB for the frequencyband to exist below the threshold value. In this regard, ˜80% of the wetsignal will be mixed with ˜20% of the dry signal. This mixed signal maybe output by the DSF 800.

In some examples, the mixing may involve equalizing one or more of thedry signal and wet signal and adjusting a phase of the dry signal and/orwet signal.

In the case that the DSF 800 is performed “in the cloud” or by thecontroller, the mixed audio signal may be sent to the audio playbackdevice for playback.

FIG. 13 is a flow chart of functions 1300 associated with the DSF. At1302, an audio signal may be received. The audio signal may be a dryaudio signal. At 1304, an envelope of the dry audio signal may becompared to a threshold of the playback device to determine a dryexcess. In some examples, the threshold may take the form of theperformance ceiling response. The performance ceiling response mayrepresent physical, electrical, and/or thermal limits of the audioplayback device, and more specifically the audio playback pipeline. At1306, a filter may be applied to the audio signal to generate anotheraudio signal. The other audio signal may be a wet audio signal. At 1308,an envelope of the wet audio signal may be compared to the threshold ofthe playback device to determine a wet excess. At 1310, based on the dryand wet excess, a dry gain may be determined for the dry audio signaland a wet gain may be determined for the wet audio signal. The gains maybe determined based on whether an envelope of the dry signal and/or wetsignal are above or below the threshold. At 1312, the wet audio signaland the dry audio signals may be mixed in proportion to respective gainsto output a mixed audio signal.

In some examples, parameters associated with the filter that is appliedmay change as a function of the dry gain and/or wet gain. For example, aquality factor (Q) of the filter may be adjusted as the dry gain rangesfrom 0 to 1 such that as dry gain approaches 1, the Q factor maydecrease. The adjustment of the filter may result in effect of thefiltering being less perceptible as the signal output by the limiterapproaches a 100% dry signal. Other parameters may also be varied suchas bandwidth and/or cutoff frequency.

FIG. 14 shows an example of the plurality of DSFs 1400 arranged in anaudio playback environment. In this arrangement, two or more DSFs may becascaded together to act as a limiter. For example, DSF 1402, DSF 1404,and DSF 1406 may be cascaded together, but more or less DSFs may becascaded together than what is illustrated. DSF 1402 may receive anaudio signal, process the audio signal, and output an audio signal toDSF 1404. In turn, DSF 1404 may receive the audio signal output by DSF1402 (e.g., mixed audio signal) and output an audio signal. In turn, DSF1406 may receive the audio signal output by DSF 1404 and output an audiosignal. This process may be repeated by one or more DSFs until the audiosignal output by the limiter is input into the audio playback pipelinefor playback.

In one example, each DSF 1402, 1404, 1406 may apply a different filterand/or in a particular order. The order in which the filters are appliedin each DSF may be chosen such that amplitudes associated with afrequency range that has larger impact on whether the audio playbackdevice stays within operational limits is reduced earlier in thecascade.

For example, DSF 1402 may apply the low band reduction filter, DSF 1404may apply the mid band reduction filter, and DSF 1406 may apply the wideband reduction filter. The low band reduction filter may be appliedfirst because an operational limit of the audio playback device tends tobe reached more readily at low frequencies with minimal perception,e.g., physical limits may be reached most readily at low frequencieswhich require large transducer movements. By applying the low bandreduction filter first, an amplitude of the audio signal may be reducedin a frequency range that yields a greatest savings in operatingexcursion. Then, for example, the mid-band reduction filter may catchexcursions not caught by the low band reduction filter. In turn, thewide-shelf filter may catch any excursions not caught by the low-bandreduction filter and the mid-band reduction filter. In other words,filters which cause less perceived bass loss may be applied earlier inthe cascade followed by filters which cause more perceived bass losslater in the cascade of DSFs.

In another example, each DSF 1402, 1404, 1406 may additionally, oralternatively apply a certain performance ceiling response. For example,DSF 1402 may apply a performance ceiling response associated with anexcursion limit and DSF 1404 may apply a performance ceiling responseassociated with a voltage limit. The cascade may facilitate applyingmultiple performance ceiling responses to the dry signal rather thanapplying an aggregated performance ceiling response that incorporatesoperational limits associated with one or more of excursion limits,voltage limits, current limits, etc.

In some embodiments, a single instance of the DSF may be used to apply aplurality of filters rather than each DSF applying a filter in cascade.In this case, the single DSF may act as a limiter.

For example, each filter, e.g., low-band, mid-band, and wide-shelf, maybe applied to a full spectrum e.g., 0 Hz to 22,050 Hz, of the dry signalto produce a corresponding wet signal at 806. Then, at 806, thecorresponding wet signals may be combined to produce a combined wetsignal from which a wet excess can determined in accordance with thefunctions described with respect to FIGS. 8 and 13. For example, thecorresponding wet signals may be averaged together to produce a singlewet signal. In this regard, a single instance of the DSF may be used toapply a plurality of filters to a dry audio signal.

In another example, a plurality of filters may be combined and appliedto a full spectrum e.g., 0 Hz to 22,050 Hz, of the dry signal yielding awet signal at 806. For example, the low and mid band reduction filtersmay be combined e.g., convolved, summed, averaged, and/or multiplied,with the mid band reduction filter to produce a combined filter to catchmost excursions in excess of the operational limit. Then, the wide shelfmay be combined to catch any excursions that the low and mid bandreduction filters missed. The combined filter may be applied to the dryaudio signal at 806. In this regard, a single instance of the DSF may beused to apply a plurality of filters to a dry audio signal.

A playback environment may have one or more audio playback devicesplaying back audio. All or a subset of the audio playback devices mayapply the described limiting. In some examples, the filters used in oneor more DSFs for each audio playback device may be the same. In otherexamples, the filters used in one or more DSFs for each audio playbackdevice may be different because each audio playback device may playdifferent spectral content. In this regard, the spectral content playedby an audio playback device may determine the filters used in one ormore DSFs and an order by which the filters are applied via a cascade ofDSFs.

For example, an audio playback device arranged as a subwoofer may outputlow frequency content in a range of 20 to 200 Hz while other audioplayback devices playing audio in synchrony with the subwoofer mayoutput audio content in a range of 200 Hz to 22,050 Hz. In this regard,a low-band reduction filter may only be used in the DSF for the otheraudio playback devices in this arrangement because there is less of achance of operational limits being reached because the audio playbackdevice arranged as a subwoofer is playing low frequency content. Thelow-band reduction filter may be sufficient to avoid the other audioplayback devices exceeding the operational limits. However, one or morefilters, e.g., the low-band, mid-band, and wide shelf, may be used in aDSF or a cascade of DSFs for the audio playback device arranged as asubwoofer. Other variations are also possible.

Additionally, or alternatively, the filters used in the one or more DSFsfor an audio playback device arranged as a subwoofer may differ from thefilters used in the one or more DSFs for other audio playback devicesnot arranged as a subwoofer. The filters may differ because theperformance ceiling responses in the devices differ. For example, theaudio playback device arranged as a subwoofer may have a higherperformance ceiling response as compared to an audio playback device notarranged as a subwoofer. This is because the audio playback devicearranged as a subwoofer is designed to output more low frequency contentas compared to an audio playback device not arranged as a subwoofer. Asa result, the filters used in the one or more DSFs for the audioplayback device arranged as a subwoofer may have differentcharacteristics, e.g., less attenuation, lower cutoff, from the filtersused by the one or more DSFs for the audio playback device not arrangedas a subwoofer.

The filters used in the one or more DSFs may also depend on whether thecontent is played by one audio playback device or a plurality of audioplayback devices. For example, a plurality of audio playback devices mayplay audio in stereo or synchronously play audio in a home theaterenvironment. In these arrangements, the low frequency content may bedistributed and played back among the plurality of audio playbackdevices. The amount of limiting needed in this case may be less, e.g.,less gain reduction, than if one or a few audio playback devices areplaying the audio. In turn, the filters used in the one or more DSFs ofthe plurality of audio playback devices playing audio may differ fromwhen one or a few audio playback devices are playing the audio in stereoor in synchrony in a home theater environment.

In other embodiments, the DSF may not apply a filter to a full spectrumof the dry audio signal, e.g., 0 Hz to 22,050 Hz. Instead, the DSF mayapply the filter to a band of the dry audio signal less than a fullspectrum. By applying the filter to less than a full spectrum, bufferrequirements may be lessened. For example, the filter, e.g., low-band,mid-band, or wide shelf, may be separately applied to the 0-20 Hz band,the 20-50 Hz band, and the 50-100 Hz band of the dry audio signal. Bandwise methods may reduce buffer requirements since a smaller spectrum isprocessed by the audio playback device at a time at an expense ofincreased latency.

In one example, each application of the filter may produce acorresponding wet signal for the dry signal for a respective band. Thewet signals for each band may be combined to generate a wet signal forthe plurality of bands, and then this wet signal and the dry signal maybe used to determine a wet gain and dry gain and the mixed signal forthe plurality of bands. In another example, after applying the filter toeach band, the wet gain and dry gain for each band may be calculated,and a mixed signal calculated for each band. The mixed signal for eachband may be then combined to generate a mixed signal for the pluralityof bands. In this embodiment, the DSF may apply a filtering betweenbands of the mixed signals to smooth transition of the mixed signalbetween bands.

In another example, a plurality of filters may be combined and thenapplied by the DSF to each band of the dry signal, yielding acorresponding wet signal. For example, the low-band reduction filter maybe combined, e.g., convolved, averaged, summed, and/or multiplied, withthe mid-band reduction filter to produce the combined filter. Then, thecombined filter may be separately applied to the 0-20 Hz band, then20-50 Hz band, and then 50-100 Hz band of the dry audio signal asdescribed above.

In yet another example, each DSF in a cascade of DSFs may apply a filterwhich filters in a certain band. For example, a first DSF may apply afilter covering a 0-20 Hz band and a second DSF may apply a filtercovering a 20-50 Hz band. The respective wet signal may be used todetermine a mixing of the wet and dry signal at each instance of theDSF. To illustrate, a dry signal may be 8 dB above the threshold in the20-50 Hz range; a first wet signal in the first DSF may be 2 dB belowthe threshold in a 20-30 Hz range; and a second wet signal in the secondDSF may be 1 dB below the threshold in the 30-50 Hz range. In this case,the first wet signal may be more heavily mixed than the second wetsignal. Other variations are also possible.

Further, examples describe a DSF which takes as input an audio signaland outputs a mixed audio signal. The DSF may be associated with othersidechains associated with audio playback including in the frequencydomain (expander, compressor, etc.) or in the time domain (reverb,delay, etc.) besides just acting as a limiter.

IV. Conclusion

The description above discloses, among other things, various examplesystems, methods, apparatus, and articles of manufacture including,among other components, firmware and/or software executed on hardware.It is understood that such examples are merely illustrative and shouldnot be considered as limiting. For example, it is contemplated that anyor all of the firmware, hardware, and/or software aspects or componentscan be embodied exclusively in hardware, exclusively in software,exclusively in firmware, or in any combination of hardware, software,and/or firmware. Accordingly, the examples provided are not the onlyway(s) to implement such systems, methods, apparatus, and/or articles ofmanufacture.

Additionally, references herein to “embodiment” means that a particularfeature, structure, or characteristic described in connection with theembodiment can be included in at least one example embodiment of aninvention. The appearances of this phrase in various places in thespecification are not necessarily all referring to the same embodiment,nor are separate or alternative embodiments mutually exclusive of otherembodiments. As such, the embodiments described herein, explicitly andimplicitly understood by one skilled in the art, can be combined withother embodiments.

The specification is presented largely in terms of illustrativeenvironments, systems, procedures, steps, logic blocks, processing, andother symbolic representations that directly or indirectly resemble theoperations of data processing devices coupled to networks. These processdescriptions and representations are typically used by those skilled inthe art to most effectively convey the substance of their work to othersskilled in the art. Numerous specific details are set forth to provide athorough understanding of the present disclosure. However, it isunderstood to those skilled in the art that certain embodiments of thepresent disclosure can be practiced without certain, specific details.In other instances, well known methods, procedures, components, andcircuitry have not been described in detail to avoid unnecessarilyobscuring aspects of the embodiments. Accordingly, the scope of thepresent disclosure is defined by the appended claims rather than theforgoing description of embodiments.

When any of the appended claims are read to cover a purely softwareand/or firmware implementation, at least one of the elements in at leastone example is hereby expressly defined to include a tangible,non-transitory medium such as a memory, DVD, CD, Blu-ray, and so on,storing the software and/or firmware.

We claim:
 1. A computing device comprising: at least one processor; atleast one non-transitory computer-readable medium; and programinstructions stored on the at least one non-transitory computer-readablemedium that are executable by the at least one processor such that thecomputing device is configured to: receive an indication of anoperational limit of a given playback device, wherein the given playbackdevice is included in a group of at least two playback devices that areconfigured for synchronous playback of audio content; receive a firstaudio signal for playback by the group of at least two playback devices;based on the operational limit of the given playback device, apply afilter to the first audio signal to generate a second audio signal; andtransmit the second audio signal to at least the given playback devicefor synchronous playback by the group of at least two playback devices.2. The computing device of claim 1, wherein the given playback device isa first playback device in the group of at least two playback devices,wherein the computing device comprises a second playback device of thegroup of at least two playback devices, and wherein the programinstructions that are executable by the at least one processor such thatthe computing device is configured to transmit the second audio signalto at least the given playback device comprise program instructions thatare executable by the at least one processor such that the computingdevice is configured to transmit the second audio signal to at least thegiven playback device via a local area network (LAN).
 3. The computingdevice of claim 2, wherein the group of at least two playback devicescomprises a third playback device, and wherein the computing devicefurther comprises program instructions stored on the at least onenon-transitory computer-readable medium that are executable by the atleast one processor such that the computing device is configured to:transmit the second audio signal to the third playback device forsynchronous playback by the group of at least two playback devices. 4.The computing device of claim 2, further comprising program instructionsstored on the at least one non-transitory computer-readable medium thatare executable by the at least one processor such that the computingdevice is configured to: after transmitting the second audio signal tothe given playback device, play back the first audio signal in synchronywith playback of the second audio signal by the given playback device.5. The computing device of claim 2, further comprising programinstructions stored on the at least one non-transitory computer-readablemedium that are executable by the at least one processor such that thecomputing device is configured to: after transmitting the second audiosignal to the given playback device, play back the second audio signalin synchrony with playback of the second audio signal by the givenplayback device.
 6. The computing device of claim 1, wherein thecomputing device is communicatively coupled to the group of at least twoplayback devices via a LAN, and wherein the group of at least twoplayback devices comprises a bonded zone that is configured forsynchronous playback of audio content in a home theater environment. 7.The computing device of claim 1, wherein the computing device is notincluded in the group of at least two playback devices that areconfigured for synchronous playback of audio content.
 8. The computingdevice of claim 7, wherein the computing device comprises a cloud-basedcomputing device that is communicatively coupled to the group of atleast two playback devices via a wide-area network (WAN).
 9. Thecomputing device of claim 1, wherein the program instructions that areexecutable by the at least one processor such that the computing deviceis configured to apply the filter to the first audio signal to generatethe second audio signal comprise program instructions that areexecutable by the at least one processor such that the computing deviceis configured to (i) apply a first filter to a first band of the firstaudio signal and (ii) apply a second filter to a second band of thefirst audio signal, wherein the first band and second band arenon-overlapping frequency bands.
 10. A non-transitory computer-readablemedium, wherein the non-transitory computer-readable medium isprovisioned with program instructions that, when executed by at leastone processor, cause a computing device to: receive an indication of anoperational limit of a given playback device, wherein the given playbackdevice is included in a group of at least two playback devices that areconfigured for synchronous playback of audio content; receive a firstaudio signal for playback by the group of at least two playback devices;based on the operational limit of the given playback device, apply afilter to the first audio signal to generate a second audio signal; andtransmit the second audio signal to at least the given playback devicefor synchronous playback by the group of at least two playback devices.11. The non-transitory computer-readable medium of claim 10, wherein thegiven playback device is a first playback device in the group of atleast two playback devices, wherein the computing device comprises asecond playback device of the group of at least two playback devices,and wherein the program instructions that, when executed by the at leastone processor, cause the computing device to transmit the second audiosignal to at least the given playback device comprise programinstructions that, when executed by the at least one processor, causethe computing device to transmit the second audio signal to at least thegiven playback device via a local area network (LAN).
 12. Thenon-transitory computer-readable medium of claim 11, wherein the groupof at least two playback devices comprises a third playback device, andwherein the non-transitory computer-readable medium is also provisionedwith program instructions that, when executed by the at least oneprocessor, cause the computing device to: transmit the second audiosignal to the third playback device for synchronous playback by thegroup of at least two playback devices.
 13. The non-transitorycomputer-readable medium of claim 11, wherein the non-transitorycomputer-readable medium is also provisioned with program instructionsthat, when executed by the at least one processor, cause the computingdevice to: after transmitting the second audio signal to the givenplayback device, play back the first audio signal in synchrony withplayback of the second audio signal by the given playback device. 14.The non-transitory computer-readable medium of claim 11, wherein thenon-transitory computer-readable medium is also provisioned with programinstructions that, when executed by the at least one processor, causethe computing device to: after transmitting the second audio signal tothe given playback device, play back the second audio signal insynchrony with playback of the second audio signal by the given playbackdevice.
 15. The non-transitory computer-readable medium of claim 10,wherein the computing device is communicatively coupled to the group ofat least two playback devices via a LAN, and wherein the group of atleast two playback devices comprises a bonded zone that is configuredfor synchronous playback of audio content in a home theater environment.16. The non-transitory computer-readable medium of claim 10, wherein thecomputing device is not included in the group of at least two playbackdevices that are configured for synchronous playback of audio content.17. The non-transitory computer-readable medium of claim 16, wherein thecomputing device comprises a cloud-based computing device that iscommunicatively coupled to the group of at least two playback devicesvia a wide-area network (WAN).
 18. The non-transitory computer-readablemedium of claim 10, wherein the program instructions that, when executedby the at least one processor, cause the computing device to apply thefilter to the first audio signal to generate the second audio signalcomprise program instructions that, when executed by the at least oneprocessor, cause the computing device to (i) apply a first filter to afirst band of the first audio signal and (ii) apply a second filter to asecond band of the first audio signal, wherein the first band and secondband are non-overlapping frequency bands.
 19. A method carried out by acomputing device, the method comprising: receiving an indication of anoperational limit of a given playback device, wherein the given playbackdevice is included in a group of at least two playback devices that areconfigured for synchronous playback of audio content; receiving a firstaudio signal for playback by the group of at least two playback devices;based on the operational limit of the given playback device, applying afilter to the first audio signal to generate a second audio signal; andtransmitting the second audio signal to at least the given playbackdevice for synchronous playback by the group of at least two playbackdevices.
 20. The method of claim 19, wherein the given playback deviceis a first playback device in the group of at least two playbackdevices, and wherein the computing device comprises a second playbackdevice of the group of at least two playback devices, and wherein themethod further comprises: after transmitting the second audio signal tothe given playback device, playing back the first audio signal insynchrony with playback of the second audio signal by the given playbackdevice.